An essential step in the fabrication of integrated circuit packages is the formation of electrical contacts to the integrated circuit chip. The chip is typically mounted on a support member, commonly termed a paddle, and electrically contacted through leads from what those skilled in the art call a lead frame. The leads extend to the outside of the package. As might be expected, several techniques have been developed for making good electrical contacts between the chip and the leads. One exemplary technique forms the contacts by wire bonding. In this technique, individual wires are attached to a lead and a corresponding site on the chip; i.e., there is one site on the chip for each lead. Another exemplary technique bonds the leads directly to solder or gold bumps on the chip. The leads are typically on a metal tape with one set of leads for each chip. The latter technique of forming the contacts can be highly automated and, in its automated form, is generally referred to by those skilled in the art as Tape Automated Bonding (TAB).
Several techniques have been developed for forming bonds, i.e. electrical contacts, between the leads and the chip. One standard technique for forming bonds to gold bumps is called thermocompression bonding. In this technique, a combination of heat and pressure is used to form the electrical contact. A solid, flat-faced object, termed a thermode, is heated and used to press the leads against the chip. The combination of pressure and thermal energy forms the electrical contacts, bonds, between the chip and the leads.
Another standard technique forms the electrical contacts by using solder bumps on the chip. The leads are first positioned over the solder bumps on the chip. A thermode is heated to a temperature which is above the melting point of the solder and brought into contact with the leads. Sufficient force is used to insure that the leads intimately contact the solder bumps. After the solder melts, cooling of the thermode beings while it contacts the leads. After the temperature of the thermode has decreased enough so that the solder has solidified, the thermode is removed from contact with the leads. The now bonded device is removed from the bonding apparatus and the process repeated with the next device.
It is desirable that the heating and cooling of the thermode proceed as rapidly as possible and thermodes are therefore designed to have minimal thermal mass. An exemplary design is called a rail thermode and has two or more rails that contact the leads. Two- and four-rail thermodes will simultaneously bond leads to two and four sides of the chips, respectively. However, rail thermodes have disadvantages as compared to flat faced thermodes. For example, they are more difficult to machine than are flat-faced thermodes and are subject to wear when forces are applied. Additionally, to minimize temperature variations at the perimeter of the rails, both the structure of the rail supports and the rail widths should vary. This, of course, makes fabrication of the thermode more difficult and also makes temperature control more difficult. It would be desirable to perform tape-automated bonding using a constant temperature flat-faced thermode.